The invention relates generally to computed tomography systems and methods, and more particularly to computed tomography systems and methods utilizing a filter.
Typically, energy-sensitive computed tomography systems employ one of two techniques: acquiring projection data using dual-energy principles, which modulate the spectrum from the X-ray tube by selecting the operating voltage of the X-ray tube or by spectral filtering techniques, or utilizing detector technology to provide energy-sensitive measurements. In one example of the former technique, data is acquired from an object using two operating voltages of an X-ray source to obtain two sets of measured intensity data using different X-ray spectra, which are representative of the X-ray flux that impinges on a detector element during a given exposure time. In general, at least one data set is then processed to represent line integrals of the linear attenuation coefficients of the object along paths of X-ray radiation from the source to the individual detector elements. The measured data that are processed are typically called projections. By using reconstruction techniques, cross-sectional images of the scanned object are formulated from the projections. Utilizing both sets of projection data acquired with different X-ray spectra, line integrals of the density distribution within the field of view of the imaging system of two chosen basis materials can be generated. By using reconstruction techniques, cross-sectional images of the density distributions for both basis materials can be formulated or the effective atomic number distribution within the field of view of the imaging system computed.
X-ray beam attenuation caused by a given length of a material, such as, but not limited to, bone or soft tissue, may be represented by an attenuation coefficient for that material. The attenuation coefficient models separate physical events that occur when the X-ray beam passes through a given length of the material. A first event, known as Compton scatter, denotes the tendency of an X-ray photon, passing through the length of the material, to be scattered or diverted from an original beam path, with a resultant change in energy. A second event, know as photoelectric absorption, denotes the tendency of an X-ray photon, passing through the length of the material, to be absorbed by the material.
Different materials differ in the scatter and absorption properties, resulting in different attenuation coefficients. In particular, the probability of Compton scattering depends in part on the electron density of the imaged particle and probability of photoelectric absorption depends in part on atomic number of the imaged material, i.e., the greater the atomic number, the greater the likelihood of absorption. Furthermore, both Compton scattering and photoelectric absorption depend in part on the energy of the X-ray beam. As a result, materials can be distinguished from one another based upon relative importance of photoelectric absorption and Compton scattering effects in X-ray attenuation by the material. A density distribution and an effective atomic number distribution may be obtained using the two sets of projection data. However, the technique has limitations due to a slow acquisition mechanism since projection data sets corresponding to two separate energy spectra from the X-ray tube must be measured.
In a latter technique, energy sensitive detectors such as, but not limited to, photon counting detectors and dual-layered detectors are used. However, at high count rates, photon-counting detectors experience charge trapping, which limits the absolute incident flux rate that may be accommodated. Furthermore, the dual-layered detectors are not cost effective since two separate detectors are needed to generate the requisite projection data.
Therefore, it is desirable to employ an energy-sensitive computed tomography system that can address one or more of the aforementioned issues.